1. Field of the Invention.
The present invention relates generally to light weight high temperature well cement compositions and methods, and more particularly, to such compositions and methods which are suitable for cementing high temperature wells containing carbon dioxide.
2. Description of the Prior Art.
In the completion of high temperature subterranean wells containing carbon dioxide, eg., geothermal wells, the use of conventional hydraulic cement compositions often results in early well failure. Because of the high static well bore temperatures involved coupled with the presence of brines containing carbon dioxide, conventional hydraulic well cements rapidly deteriorate due to alkali carbonation, especially sodium carbonate induced carbonation. In geothermal wells which typically involve very high temperatures, pressures and carbon dioxide concentrations, conventional well cement failures have occurred in less than five years causing the collapse of the well casing.
It has heretofore been discovered that a cement material known as calcium phosphate cement formed by an acid-base reaction between calcium aluminate and a phosphate-containing solution has high strength, low permeability and excellent carbon dioxide resistance when cured in hydrothermal environments. However, calcium phosphate cement has a relatively high density, eg., a density in the range of from about 15 to about 17 pounds per gallon, which is too high for geothermal applications. That is, in geothermal wells the hydrostatic pressure exerted by the high density calcium phosphate cement often exceeds the fracture gradients of subterranean zones penetrated by the well bore which causes the formation of fractures into which the cement is lost. While calcium phosphate cements have been developed which include hollow microspheres and as a result have densities of about 10 pounds per gallon, such light weight compositions are relatively expensive and the presence of the microspheres in the cured cement reduces its compressive strength.
Thus, there is a need for improved less expensive well cement compositions useful in cementing high temperature wells containing carbon dioxide.